I have found the issue. Somehow or another the mountpoints changed after a reboot. I didn't notice for the two empty drives because there was no data to verify and the directories were still present. The 3 drives were actually mounting to /media/pi/DriveX instead of /media/DriveX.
I think...
In the past 24 hours I've given myself a crash course in adding and sharing external drives in Linux (Raspbian). After relying on forums like this one I was able to piece together how to accomplish this since I'm new to Linux. I got 1 external drive set up and visible/writable on my Windows PC...